1. Estimating TP – models and pit-falls
Olivine-liquid equilibria
Roeder & Emslie 1970 Contrib. Mineral. Petrol., 29,
Dungan & Rhodes 1978 Geophys. Res. Lett. 5,
Mg# Olivine
Olivine is in equilibrium with a host liquid
Composition of olivine varies systematically with temperature
Based on partitioning of Fe and Mg into olivine during crystallization
Mg# host-rock

2. Equilibrium Olivine Herdubriedatogl HBT-10 Crystallization T not TP
MgO
FeO
Eqm Ol
Temp (°C)
Rock
17.5
9.6
90.5
1390
Olivine
49.1
9.2
15.1
89.3
1346
48.2
10.3
9.9
9.3
85.9
1230
45.7
13.4
8.0
10.4
79.3
1185
42.9
16.8
L+Ol
Mantle olivine
Fractional crystallization
L+Ol+Plag
L+Ol+Pl+Cpx

3. Equilibrium Olivine addition
(e.g. Larsen & Pedersen 2000)
Add or subtract olivine to or from a magma that has crystallized only olivine, until olivine in magma is in equilibrium with mantle olivine
MgO a function of T
FeO a function of P
Fo84
Fo88
Fo92
solidus
L+Ol
Is the olivine in equilibrium with the whole rock or glass?
Is the calculated primary magma consistent with melting of peridotite under the estimated T-P conditions?
TP=1500°C
TP=1400°C

4. Olivine compositions – West Greenland/ Baffin picrites
Normal distribution
Awful distribution
Eqm olivine
Eqm olivine
CUMULATES

5. Accumulative olivine (disequilibrium addition)
L+Ol →L+Ol+Pl+Cpx
Cotectic compositions i.e more than 1 phase crystallizing
Fo84
Fo88
Fo92
Add olivine Fo88
EPR
Normal distribution
Eqm olivine
TP=1500°C
TP= °C
TP=1350°C

6. Other major element considerations
Permitted range primary magmas
L+Ol  L+Ol+Pl  L+Ol+Pl+Cpx
Consistent with melting of mantle peridotite at TP~1350°C
Not consistent with any primary magmas from mantle peridotite
Not consistent with any primary magmas from mantle peridotite

7. Test…………
Is it possible to generate a picrite by addition of olivine to a cotectic glass to yield a high mantle potential temperature?
Use cotectic MORB glasses which should have been formed of part of a fractionation series from ambient TP (~1350°C). Siqueiros Fracture Zone.
Add olivine of Fo88 (and other Fo) until the composition of a Baffin Island picrite is acheived (~20.0 wt% MgO in the whole-rock)

8. Accumulate Fo88
Start with EPR glass samples along the cotectic and add olivine Fo88
Generate an artificial magma with same MgO as picrite (~20.0 wt%) containing accumulative olivine
Calculate model primary magma compositions
Large range of possible TP
Examine CaO-MgO……………..

9. Accumulate Fo88
Model
Cumulates
Fe3+/FeT~0.05
West Greenland
For Fo88 not all the model primary magmas are suitable candidates for derivation from mantle peridotite
Those that are not are too CaO deficient
A result of fractionation of Ol+Pl+Cpx
1449°C
1462°C
higher Fo
But – two possible solutions with variable TP of °C
uncertainty in the determination of the peridotite solidus is ±42°C
Vary Fo content of olivine
Lower Fo
Fe3+/FeT~0.09

10. Melting column considerations
MgO isopleths
TP °C 13 15 17 19
lithosphere
1350°C
1450°C
1550°C
SQFZ
Melt-fraction
F ~ 0.3
harzburgite
spinel
peridotite
Disko
Olivine
cumulates
garnet
peridotite
Hawaii
F=0

11. Mixing liquids
Mixing a MORB primary magma (~12wt% MgO) formed at TP~1350°C with cotectic liquids from a MORB fractionation series over the range 5-10 wt% MgO
Model
Cumulates
CaO deficient
West Greenland
5%MgO 6%
TP elevation, maximum 1458°C 9%
L+Ol+Pl+Cpx OK
L+Ol
Primary

12. Conclusions
For PRIMELT3, it is possible to generate elevated TP from MORB cotectic glasses by addition of disequilibrium olivine (olivine accumulation) up to a maximum of ~1460°C
But - MgO contents in olivine cumulates have to be <~16 wt % otherwise CaO is too deficient to give sensible temperature estimates
At <~8.5 wt% MgO in the cotectic liquid, accumulation to picritic compositions becomes untenable because of CaO deficiency in the melt
Other methods (e.g. simple olivine addition to glass) have no upper limits to TP and are constrained only by assumptions as to mantle olivine Fo content.
In many cases, it is not possible to generate the calculated TP and olivine- spinel equilibrium T by simple addition of olivine to a cotectic melt derived from peridotite at ambient TP

13. Conclusions - Additional T issues – source lithology
A pyroxenite component in the upper mantle might provide an explanation for both the olivine and TP
Some pyroxenite melts (experiments) can result magmas that mimic the compositions of peridotite-derived magmas – they are not necessarily CaO deficient as commonly assumed
Pyroxenite has a different melting behaviour from peridotite
More melt production at the same T
Garnet stable to shallower depths (1.7 GPa) in pyroxenite compared to peridotite allows for LREE-enrichment at lower pressures than required by garnet peridotite (≥2.7 GPa)

14. Herdubreidatogl
W Greenland
disequilibrium Ol-Gl
Olivine and glass as close to equilibrium as can be found
Tcryst 0GPa Fo86.5 TP
Method
1265 ~
Ol-Gl
1240
Ol-Sp
~1250
1498
Primelt3 Gl
1492
Primelt3 WR
Fe3+/FeT=0.05
Applying the Putirka Ol-Gl geothermometer gives a crystallization temperature of 1263±28°C
Samples from the same site have olivine-spinel equilibration Tcryst of 1241±22°C
PRIMELT3 on glasses ( wt% MgO) gives TP=1498[±42]°C at Fo91.4
PRIMELT3 on whole rocks ( wt% MgO) gives TP=1492[±42]°C at Fo91.4
This is equivalent to Tcryst~1250°C at Fo86.5, the same as the glasses